The demands of mass production of metal parts for appliances, vehicles and machines of all types has driven the technology of powder metallurgy ever since it was discovered that a mixture of the appropriate finely divided metal particles in the form of a powder, mixed with a binder and lubricant, each also in finely divided form, could be compacted and sintered to yield an article of arbitrary shape and size. In the art, “powder metal mixture” refers to a mass of particles each of which is less than 150 μm (micrometers or microns) in average equivalent diameter (“avg. equiv. dia.”), preferably its largest dimension, the metal particles typically being largest, preferably having an avg. equiv. dia. of less than 75 μm, the smallest metal particles having an avg. equiv. dia. of about 25 μm, and the non-metallic additives typically having an avg. equiv. dia. less than 50 μm. Because powder metal particles may be irregular in shape, the equivalent diameter of a particle refers to its diameter had it been a sphere of equal volume. Depending upon the physical properties demanded of the sintered or sintered and heat treated ferrous article, a wide array of powder additives are added. The most common of such additives are a lubricant and/or binder followed by graphite and metals such as nickel, copper, molybdenum, manganese, chromium, cobalt and/or an organometal or metal compounds such as sulfides, phosphides, and the like which become alloyed with the powder metal when sintered and/or heat treated. The term “powder metal mixture” in this invention refers to a mixture of ferrous metal particles in which iron (Fe) is present in an amount greater than 90 per cent, the remaining ingredients being additives such as a lubricant, optionally a binder which may be the same as the lubricant or different, and alloying ingredients such as graphite and metals, each present in an amount less than 2 per cent by weight (% by wt) of the total mixture including the powder. The term “lubricant” refers to a powder of particles no dimension of which is more than about 100 μm, and typically having an avg. equiv. dia. in the range from about 5 μm to 25 μm; in this invention the lubricant is modified to consist essentially of a lubricant powder such as is conventionally used to make a compact mass of powder metal, blended with fragmented cellulose fibers having an average length less than 150 μm, preferably less than 20 μm, and a diameter in the range from about 1 μm to 20 μm.
The overriding criterion for a practical powder mixture is its homogeneity without which there would be an unacceptable variance in composition of a compacted metal part, not only from part to part, but within a part itself. The term “part” is used interchangeably with the more formal term “article”. Such homogeneity encompasses not only the distribution of particles within a mass of unit volume of powder but the bulk density (measured as “Hall apparent density”) and flow characteristics (measured as “Hall flow rate”) of the powder mixture. The apparent density is the mass of a unit volume of non-compacted powder. Hall apparent density is measured as set forth in ASTM B-212 (Metal Powder Industries Federation “MPIF” test method 04 in “Standard Test Methods for Metal Powders and Powder Metallurgical Products). The flow rate is quantified as the time required for a powder of standard weight to flow through a Hall flow meter. The Hall flow rate is dictated by ASTM B-213 (MPIF test method 03). A variation in bulk density and flow will result in a variation in the “fill”, which is the amount of powder mixture filled in a die cavity before the mixture is compacted, and the dimensions of the compacted part. To a lesser extent, a lack of homogeneity is reflected in variations in green strength of a compacted part particularly in sensitive portions of the molded part, such as the teeth of a gear.
Mainly because adequate green strength is obtained by increasing compacting pressure, green strength as a serious problem attracts attention only in those instances where compaction, or molding pressure is already so high as to shorten the useful life of a die noticeably and/or a worrisome number of compacted parts are damaged when being ejected from the die, or being automatically conveyed to a sintering oven under conditions which cannot preclude the green compacts from being impacted, even if not forcefully.
Green strength is measured as pressure required to break an unsintered compact (a standard rectangular bar) as set forth in ASTM B-312 (MPIF test method 15).
Though the terms “binder” and “lubricant” are used as if to specify different functions in a powder metal blend to be compacted, in practice, the same function may be discharged by a single ingredient, though each function may be to a greater or lesser extent than when discharged by different highly specialized materials. In particular, ethylene-bis-stearamide (“EBS”) is sometimes referred to as a “binder” though it may also function as a lubricant, and metal soaps and waxes are typically referred to as “lubricants” though they may also function as binders. The term “blend” refers to a ferrous metal powder including all ingredients essentially homogeneously dispersed and in condition to be compacted. By “ferrous” metal powder is meant one in which the metal particles contain predominantly the element iron (Fe), typically at least 75% Fe. The binder binds particles of graphite and alloying components to the surface of metal particles. The lubricant reduces friction generated when the powder is subjected to shear, or, stressed; thus, metal powder with lubricant particles flows more easily than without the lubricant; and, a powder mixture with lubricant added to the metal particles may be compacted in a die under pressure and, if the compact has sufficient green strength, ejected from the die with less wear and tear on the die parts.
Typically, the green article is then sintered. The strength of the sintered bar is measured as “transverse rupture strength” (“TRS”) using a standard TRS fixture as described in ASTM B-528 (MPIF test method 41). If the TRS is satisfactory, the tensile strength will generally be satisfactory. Depending upon the composition of the mixture of powder metal and additives from which the article is molded, and its end use, it may be subjected to further processing steps such as sizing/coining, resintering, heat treating, and others.
Recognizing that the component powders of metal, binder and/or lubricant and one or more additional additives differ in size, density and shape, the problem of homogeneity is minimized by choosing particles of comparable size and shape and thoroughly mixing the various particles before using the powder mixture. In this framework it is evident that any thought of mixing an organic fiber of vegetable material with the other particles, no matter how beneficial the fiber might otherwise be, will be quickly dispelled.
Moreover, to date, comminuted cellulose fibers have been available in an average length no shorter than 70 μm because attempting to comminute them further typically results in forming a fibrous compacted matte. Though the length of such comminuted fibers is in the same size range as the average equivalent diameter of metal particles used in a compactable powder mixture, addition of such fibers in an amount as little as 2% by weight of the total powder mixture, results in unacceptable bulk or apparent density and flow characteristics if the mixture is to be used in the mass production of compacted and sintered parts. The poor physical properties of a powder metal mixture containing fibers 150 μm and longer is attributable to the volume the fibers occupy and the irregularity of their individual shapes.
On the other hand, it is well recognized that the increased surface area contributed by the smaller particles in a powder mixture relative to the area contributed by the larger ones, impairs the “flowability” or flow characteristics of the powder mixture, resulting in a longer time required to fill a die and additional risk of non-homogeneity in compacted parts. Though such flowability is not a problem when compacting a dozen parts or so in a laboratory, the problem may be critical in a production facility where the number of parts which can be produced per unit time is a deciding factor.
Another serious problem which has loomed large in recent years is the extent of “dusting”, particularly of graphite, and the harmful side effects of vaporizing zinc stearate, commonly used as a lubricant, during sintering. To cope with the latter problem, particularly having to clean sintering furnaces and their flues, the art is proliferated with disclosures of numerous other lubricants including waxes and metal soaps. To minimize or eliminate the use of zinc stearate, polyethylene oxide in combination with an oligomeric amide is disclosed in U.S. Pat. No. 6,511,945; and EBS or a polycarboxylic acid amide wax is used as a binder, but making a homogeneous powder mixture typically requires heating the wax to distribute it uniformly as a coating on the metal particles, as disclosed in U.S. Pat. No 5,480,469 to Storstrom et al. and U.S. Pat. No 6,573,225 to Vidarsson et al. respectively. To improve lubrication, U.S. Pat. No. 6,413,919 uses a combination of two well-known lubricants, each effective in its own right, one a fatty acid mono- or bis-amide, e.g. EBS, the other a metal soap, e.g. zinc stearate, and relies upon processing the mixture to form a core of one lubricant coated with the other. Though cellulose derivatives are broadly suggested as binders (see U.S. Pat. No. 5,480,469) and cellulose ester resins and hydroxyalkyl cellulose resins have also been suggested (see U.S. Pat. No. 6,573,225) it is evident that these compounds are physically and unrelated to cellulose fibers and have no analogous chemical properties.